Petroski on Engineering: Design Begets Design

As construction progressed, the all-steel structure drew increasing attention and visitors from both the engineering community and the general public. Because of the legacy of the Tay Bridge and the oddness of the Forth's form, Baker was prompted to prepare a public lecture in which he could explain that the new bridge was not only distinct from the Tay but also much better able to withstand the force of the wind. In the course of preparing his lecture on the unusual design, he found that he needed to solve another design problem: how to develop a visual model of the great cantilever that could be easily grasped by a general audience. It was here that playfulness reigned.

In preparing his lecture, Baker found himself looking for a way to make even non-specialists in the audience appreciate the "true nature and direction of the stresses on the Forth Bridge." After consulting with colleagues on the construction site, a "living model" of the cantilever structure was conceived as an illustration for the lecture. The tableau has also been described as a "human model," because in place of some of the structural components of the bridge were people who could experience directly the forces involved.

Fortunately, one of the assistant engineers of the project, who was also its official photographer, was present as the design of the model evolved. He captured not only the final version but also some early, less effective versions of the model. The series of photographs thus shows how the model progressed from a concept to a polished work of anthropomorphic structure.

The essential structure of the bridge consists of distinct steel towers from which cantilevers reach out toward each other and support a central spanning girder. To counterbalance the weight of these cantilevers plus the suspended central span between them -- and the trains that run along the whole assembly -- opposing cantilevers are anchored to end piers of massive masonry construction.

The human model that mimicked the actual structure consisted of a pair of chairs on each of which a person sat. These represented the steel towers. Each person held a wooden strut, one end of which was wedged between the person's thigh and the chair seat and the other end was grasped by the person's outstretched arm. Together, this arrangement represented a cantilever. The end of the counterbalancing cantilever was attached by a rope to a pile of bricks that sat on the ground. From the ends of the inside cantilevers was suspended a swing seat, representing the suspended central girder. When a person sat in this elevated swing seat, he represented a train on the bridge and so completed the model.

The seated persons felt a pull in their arms and also felt the strut pushing into their thigh, thereby experiencing both the tension in the top chord of the cantilever truss and the compression in the bottom one. The seated persons also felt their torso being pushed down onto the seat of the chair, thereby experiencing the great compression that the piers felt under the weight of the structure and its loads. The audience viewing a lantern slide of the model demonstration could feel the same forces. The model was as beautifully designed as the bridge itself.

The story of that demonstration of the concept of the bridge is really interesting. Before the days of electronics to be able to get such a concept across was a difficult proposition. The solution is very good and filled the bill well.

This is definitely a great way to illustrate concepts of stress and strain! I'll have to keep this technique in mind when trying to explain mechanical problems. By the way, here is a picture (from Wikipedia):

What I really like about the human model described here is that the humans could feel the tensile and compressive forces, rather than just imagine them. Seems like it would be a great exercise for engineering students.

Using words to let us paint our own mental picture is to me the essence of teaching. Teach us to read and imagine, then present a problem and describe a solution and let our minds create the picture and fill in the details. I read the article and had a pretty good concept of what was being described, when I saw the photo, it was obvious what had been described and the details clicked immediately into place. I think those of us who learned to read before there was television may be luckier than our children who had all the solutions presented visually before they developed the ability to imagine. I always enjoy your articles Professor Petroski.

Ken, You are absolutely correct. A picture is worth a lot of words, but it was my understanding that my column was not to be illustrated. Thanks to Mr. Palmer for inserting the classic photo into his comment. HP

This article may not be illustrated, but "Engineers of Dreams" is, and I remember the image shown below is in that book. I've kept it on my shelf waiting for my son to be old enough to understand and appreciate it as he heads towards an engineering career.

Sorry, Mr. P, I had no idea there was any such limitation, it seems I see photo's here quite regularly. Editor?

Bob from Maine, I'm quite proud of my ability to describe things accurately, but like most engineers regardless their artistic ability, I am always sketching stuff during discussions. One wouldn't commit schematics or drawings to a written description. Imaginations are way too variable to assure our minds are on the same page.

I'd seen this fantastic photo some time ago too, (Perhaps in Mr. P's book) and although it all sounded familiar, I still didn't put it together.

An interesting title, although my experience is that designs build on designs. It is often easier to imagine an improvement of some kind when looking at a design then when looking at a blank sheet. Even an unworkable dsign can serve as a basis for ides to produce a much better design. At least that is often what I see. Of course, sometimes we can only guess about what the main goal of a design was. Some designs seek to minimize the required accuracy of components, others strive for the maximum robustness, and it is obvious that many designs are optimized for minimum initial cost, with little attention given to other variables.

So every design can serve as a basis for additional designs, optimized for some particular variable parameter.

Design does beget design. In the article, the design of the demo was a result of the design of the bridge. That is the circle of life of engineering. If you design an end item, there are many other designs that are necessary to make your assembly a reality. If the component requires plastic parts, someone has to design the molds for the part. Someone else handles the design of the molding mahcine to run the molds. This goes on up and down the chain keeping engineers in business.

The comment about secondary required designs brings out the old comment "Nothing is ever simple". The truth is that usually an over-all design does require a lot of little designs, such as nuts and bolts. The good news is thgat we don't need to design those parts new each time.

Dr. Petroski--absolutely fascinating article. I love these stories that give background and weave into the narrative personal history regarding the engineer(s) doing the work. We sometimes forget that many many great engineering designs were accomplished with slide rules, pencils and erasers. (Big erasers at that.) Really demonstrates how far we have progressed with technology. I wonder where we will be fifty years from now. Again, many thanks.

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Load dump occurs when a discharged battery is disconnected while the alternator is generating current and other loads remain on the alternator circuit. If left alone, the electrical spikes and transients will be transmitted along the power line, leading to malfunctions in individual electronics/sensors or permanent damage to the vehicle’s electronic system. Bottom line: An uncontrolled load dump threatens the overall safety and reliability of the vehicle.

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To those who have not stepped into additive manufacturing, get involved as soon as possible. This is for the benefit of your company. When the new innovations come out, you want to be ready to take advantage of them immediately, and that takes knowledge.

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